1. Field of the Invention
The present invention relates generally to chairs and particularly to chairs with back rests which are adjustable to individual spinal curvatures.
2. Prior Art
In the human population the range of spinal length and curvature is, for practical purposes, infinite. Also, it is clear that the pressure exerted on an individual's back when in a seated position by the seat's back rest can produce discomfort and even injury to, for example, the lower back region, due to lack of conformity between the individual's spinal curvature and the curvature of the seat's back rest. There has always been, therefore, a need for a chair with an adjustable back rest to accomodate to some degree this variations in spinal length and curvature.
In fact, chairs with back rests which are adjustable in contour are known. See, for example, U.S. Pat. Nos. 2,549,902; 2,550,831; 3,288,525; 4,313,657; and 4,347,840. Of the chairs disclosed in these patents, that disclosed in 2,549,902 has perhaps the greatest degree of adaptability due to the provision of the rows of individually adjustable heads each of which covers a small area of the back covering of the chair. The individual adjustment allows for a variety of contours. Due to the limited area of application of the individually adjustable heads, however, the necessary support between adjacent heads is unsatisfactory since the covering material of the back rest, which is necessarily flexible, does not ordinarily have sufficient inherent stiffness to effect a proper load distribution of the load applied by the individual heads between adjacent heads, resulting in "soft-spots". If the material is designed to have necessary stiffness, then the effect of the loads applied by the individually adjustable heads for contour control is correspondingly dimished or even lost. Moreover, the individual heads tend to produce a relatively concentrated load, the cumlative effect of which is to produce occupant discomfort.
The theoretical rationale for maintaining an individual's standing spinal curvature, i.e., lordosis and kyphosis during sitting is presented by Anderson et al, 1979 Spine, Vol. 4, Number 1, in their paper delineating spinal shape. Using X-ray analysis they quantified the mean reduction in lumbar lordosis during sitting as 38.degree.. Therefore, an effective sitting modality should re-establish the existing lumbar lordosis. This would have the benefit of reducing the tensile stresses in the posterior annulus fibers during sitting. Futhermore, it is essential that the apex of the spinal curvature coincide with the apex of the seat curvatures to reinstitute the standing curvatures with a minimum of stress concentration. This necessitates longitudinal and transverse plane adjustments of the backrest position.
There exists, therefore, a need for a chair with a back rest whose contour can be adjusted to suit the particular needs or posture of the occupant such that a substantially uniform load distribution without any effective "soft-spots" results.
This would provide maximum distribution of stress over the entire length of the spine, thereby reducing compressive force within the intervertebral disc and resulting strain in the annulus fibrosis.